1. Field of the Invention
The present invention relates to a liquid crystal display (LCD), and more particularly, to a thin film transistor (TFT) of an LCD device and a fabrication method thereof.
2. Description of the Related Art
Generally, an LCD operates using the optical anisotropy and the polarization of liquid crystal molecules. Since the liquid crystal molecules have an elongated thin structure, an arrangement of liquid crystal molecules having an orientation can be formed. The direction of the arrangement of liquid crystal molecules can be controlled by artificially applying an electric field. Therefore, it is possible to display image information by controlling the direction of the arrangement of the liquid crystal molecules so as to modulate light polarized by the optical anisotropy. Liquid crystal molecules can be classified into positive liquid crystal molecules having positive dielectric anisotropy and negative liquid crystal molecules having negative dielectric anisotropy according to their electrical characteristics. In the liquid crystal molecules having positive dielectric anisotropy, a longer axis of the liquid crystal molecule is aligned in parallel with the direction in which an electric field is applied. On the contrary, in liquid crystal molecules having negative dielectric anisotropy, a longer axis of the liquid crystal molecule is aligned in a direction perpendicular to the direction in which an electric field is applied.
An active matrix-type LCD includes pixel electrodes arranged in a matrix form that are connected with the TFTs. The active matrix-type LCD has a high resolution and an excellent moving-image displaying performance and is thus widely used. The structure of a liquid crystal panel, which is a part of a liquid crystal display device, will be described below.
FIG. 1 is an exploded perspective view of a related art LCD. Referring to FIG. 1, the related art LCD has a color substrate 7 including a black matrix 6, a color filter 8 having sub-color-filters (red, green, and blue), an upper substrate 5 and a transparent common electrode 18 on the color filter. The related art LCD also includes an array substrate 11 having a lower substrate 22 with gate lines 13 and data lines 15 that cross each other and define pixel regions P. Each pixel region P has a pixel electrode 17 and a switching device T, such as a TFT. The pixel electrode 17 is formed of transparent conductive metal having excellent light transmittance, such as indium-tin-oxide (ITO). The TFTs (T) are arranged in a matrix form on the array substrate and are formed adjacent to where the gate lines 13 and data lines 15 cross each other. Liquid crystal molecules 14 fill a space between the upper substrate 5 and the lower substrate 22.
In the above-described related art LCD, the liquid crystal molecules 14 disposed on the pixel electrode 17 are aligned by a signal applied through the TFT. Light passing through the liquid crystal layer is controlled according to the alignment degree of the liquid crystal layer, so that an image can be displayed. That is, the TFT performs the function of switching on and switching off the respective pixels of the related art LCD. To maintain good image quality, it is required that the TFTs have the same electrical characteristics as well as the same fast switching speed.